The present invention relates to a color image forming apparatus using a photoreceptor drum or a photoreceptor belt on the surface of which a toner image is formed, and specifically to a color image forming apparatus by which a color image can be recorded other than a black toner image when color developing units are optionally equipped to the apparatus.
Conventionally, the following is known: a plurality of developing units are provided around the photoreceptor drum or photoreceptor belt in an image recording apparatus having the photoreceptor drum or photoreceptor belt. Specifically, an apparatus in which the photoreceptor belt is used is suitable for color image recording, for which many developing units are necessary. Because the developing units can be provided in almost the same condition. In this case, a black toner image and a color image can be recorded when 4 developing units are provided in which yellow, magenta, cyan and black toners are respectively loaded. In the conventional color image forming apparatus described above, in order to easily replace 4 developing units or in order to easily supply toners to the developing units, 4 developing units can be attached to and detached from the apparatus together with the photoreceptor drum or photoreceptor belt, or 4 developing units can be integrally attached to and detached from the apparatus independently from the photoreceptor drum or photoreceptor belt. However, when only a black toner image is recorded, developing units other than that for black toner are surplus. In order to replenish the frequently used black toner, other developing units should be detached from and attached to the apparatus together with the developing unit for black toner, which is ineffectual and can cause problems.